[gdbsupport] Add parallel_for_each_debug

[gdbsupport] Add parallel_for_each_debug

Add a parallel_for_each_debug variable, set to false by default.

With an a.out compiled from hello world, we get with
parallel_for_each_debug == true:
...
$ gdb -q -batch a.out -ex start
  ...
Parallel for: n_elements: 7271
Parallel for: minimum elements per thread: 10
Parallel for: elts_per_thread: 1817
Parallel for: elements on worker thread 0       : 1817
Parallel for: elements on worker thread 1       : 1817
Parallel for: elements on worker thread 2       : 1817
Parallel for: elements on worker thread 3       : 0
Parallel for: elements on main thread           : 1820

Temporary breakpoint 1, main () at /home/vries/hello.c:6
6         printf ("hello\n");
...

Tested on x86_64-linux.